THE UNIVERSE
ANATOMY OF THE UNIVERSE
....................................
10
GALAXIES
.................................................................
12
THE MILKY WAY
....................................................
14
NEBULAE AND STAR CLUSTERS
... ............ ........... . .....
16
STARS OF NORTHERN SKIES
......................................
18
STARS OF SOUTHERN SKIES
......................................
20
STARS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
SMALL STARS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
MASSIVE STARS
........................................................
26
NEUTRON STARS AND BLACK HOLES
........................
28
THE SOLAR SYSTEM
.................................................
30
THE SUN
..................................................................
32
MERCURY
.................................................................
34
VENUS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
THE EARTH
..............................................................
38
THE MOON
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
MARS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
JUPITER
....................................................................
44
SATU RN
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
URANUS
...................................................................
48
NEPTUNE AND PLUTO
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
ASTEROIDS, COMETS, AND METEOROIDS
..................
52
THE UNIVERSE
10
Fireball of rapidly
expanding, extremely
hot gas lasting about
one million years
Anatomy of the universe
THE UNIVERSE CONTAINS EVERYTHING that exists, from the tiniest subatomic particles
to galactic superclusters (the largest structures known). No one knows how big
the universe is, but astronomers estimate that it contains at least 125 billion
galaxies, each comprising an average of 100 billion stars. The most widely
accepted theory about the origin of the universe is the Big Bang
theory, which states that the universe came into being in a huge
explosion—the Big Bang—that took place between 10 and 20
billion years ago. The universe initially consisted of a very
hot, dense fireball of expanding, cooling gas. After about
one million years, the gas began to condense into
localized clumps called protogalaxies. During the
next five billion years, the protogalaxies continued
condensing, forming galaxies in which stars were
being born. Today, billions of years later, the
universe as a whole is still expanding, although
there are localized areas in which objects are
held together by gravity; for example, many
galaxies are found in clusters. The Big Bang
theory is supported by the discovery of faint,
cool background radiation coming evenly from
all directions. This radiation is believed to be
the remnant of the radiation produced by the
Big Bang. Small “ripples” in the temperature of
the cosmic background radiation are thought
to be evidence of slight fluctuations in the
density of the early universe, which resulted
in the formation of galaxies. Astronomers do
not yet know if the universe is “closed,” which
means it will eventually stop expanding and
begin to contract, or if it is “open,” which
means it will continue expanding forever.
FALSE-COLOR MICROWAVE MAP OF
COSMIC BACKGROUND RADIATION
Pink indicates “warm
ripples” in background
radiation
Pale blue indicates
“cool ripples” in
background radiation
Low-energy
microwave radiation
corresponding to -454°F (-270°C)
High-energy gamma
radiation corresponding
to 5,400°F (3,000°C)
Red and pink band
indicates radiation
from our galaxy
Deep blue indicates
background radiation corresponding to -454°F
(-270°C); (remnant of the Big Bang)
ANATOMY OF THE UNIVERSE
11
Spiral galaxy
containing gas,
dust,and young stars
Irregular galaxy
Elliptical galaxy
containing old stars
and little gas and dust
Cluster of
galaxiesheld
together by gravity
Universe today
(13–17 billion years
after Big Bang)
THE JEWEL BOX
(STAR CLUSTER)
THE SUN
(MAINSEQUENCE STAR)
EARTH THE MOON
NGC 6822
(IRREGULARGALAXY)
THE ROSETTE NEBULA
(EMISSION NEBULA)
Elliptical
galaxy in
which stars
form rapidly
Dark cloud
(dust and gas
condensing
to form a
protogalaxy)
NGC 4406
(ELLIPTICALGALAXY)
NGC 5236
(BARRED SPIRALGALAXY)
FALSE-COLOR IMAGE
OF 3C273 (QUASAR)
CLUSTER OF
GALAXIES IN VIRGO
OBJECTS IN THE UNIVERSE
Protogalaxy
(condensing gas cloud)
Universe about five
billion years after
Big Bang
Galaxy spinning and
flattening to become
spiral shaped
Quasar (probably the center
of a galaxy containing a
massive black hole)
ORIGIN AND EXPANSION OF THE UNIVERSE
THE UNIVERSE
12
Central region
containing old
red giants
Less densely
populated region
Neighbouring galaxy
Tarantula Nebula
Dust cloud obscuring
light from stars
Globular cluster
containing very
old red giants
Emission nebula
Glowing nebula
in spiral arm
Spiral arm
containing
young stars
Galactic nucleus
containing
oldstars
Dust in spiral
arm reflecting
blue light from
hot young stars
Hot, ionized
hydrogen gas
emitting red light
Dust lane
Light from stars
OPTICAL IMAGE OF NGC 2997 (SPIRAL GALAXY)
Galaxies
A GALAXY IS A HUGE MASS OF STARS, nebulae,
and interstellar material. The smallest
galaxies contain about 100,000 stars, while
the largest contain up to 3 trillion stars.
There are three main types of galaxy,
classified according to their shape: elliptical,
which are oval shaped; spiral,
which have arms spiraling
outward from a central bulge (those whose arms
spiral from a bar-shaped bulge are called spirals);
and irregular, which have no obvious shape.
Sometimes, the shape of a galaxy is distorted by
a collision with another galaxy. Quasars (quasi-
stellar objects) are thought to be galactic nuclei
butare so far away that their exact nature is still
uncertain. They are compact, highly luminous
objects in the outer reaches of the known
universe: while the farthest known “ordinary”
galaxies are about 12 billion light-years away, the
farthest known quasar is about 13 billion light-
years away. Active galaxies, such as Seyfert
galaxies and radio galaxies, emit intense radiation.
In a Seyfert galaxy, this radiation comes from the
galactic nucleus; in a radio galaxy, it also comes
from huge lobes on either side of the galaxy. The
radiation from active galaxies and quasars is
thought to be caused by material falling into
central black holes (see pp. 28-29).
OPTICAL IMAGE OF NGC 4486
(ELLIPTICAL GALAXY)
SOMBRERO,
ASPIRAL GALAXY
OPTICAL IMAGE OF LARGE MAGELLANIC
CLOUD (IRREGULAR GALAXY)
GALAXIES
13
Dust lane crossing
elliptical galaxy
Light from
old stars
Galactic nucleus
containing
powerful source
of radiation
Red indicates
high-intensity
radio waves
Blue indicates
low-intensity
radio waves
Radiation from
galactic nucleus
Outline of
optical image
of Centaurus A
Yellow indicates
medium-intensity
radio waves
Radio
lobe
Radio
lobe
Quasar nucleus
White indicates high-
intensity radio waves
Radiation from jet
of high-energy
particles moving
away from quasar
Blue indicates
low-intensity
radio waves
Nebula in
spiral arm
Compact nucleus
emitting intense
radiation
Spiral arm
Blue indicates low-
intensity radiation
Red indicates
medium-intensity
radiation
Spiral arm
distorted by
gravitational
influence of
smaller galaxy
Large spiral
galaxy
Smaller galaxy
colliding with
large galaxy
Yellow indicates
high-intensity
radiation
FALSE-COLOR OPTICAL IMAGE OF NGC 5754
(TWO COLLIDING GALAXIES)
OPTICAL IMAGE OF NGC 1566
(SEYFERT GALAXY)
FALSE-COLOR RADIO IMAGE OF 3C 273 (QUASAR)
FALSE-COLOR RADIO
IMAGE OF CENTAURUS A
OPTICAL IMAGE OF CENTAURUS A
(RADIO GALAXY)
THE UNIVERSE
14
Disk of spiral arms
containing mainly
young stars
Central bulge containing
mainly older stars
Halo containing
oldest stars
Nucleus
100,000 light-years
Milky Way
(theband of light
that stretches across
the night sky)
Galactic
plane
Light from stars
and nebulae in the
Perseus Arm
Polaris (the North
Star), a blue-green
variable binary star
Emission
nebula
Sagittarius
Arm
Orion Arm
(Local Arm)
Pleiades
(the Seven Sisters),
an open star cluster
Andromeda Galaxy, a spiral
galaxy 2.2 million light-years
away; the most distant object
visible to the naked eye
Central bulge
Nucleus
Perseus Arm
Crux-Centaurus
Arm
Location of
solar system
Patch of dust clouds
Dust in spiral
arm reflecting
blue light from
hot young stars
The Milky Way
THE MILKY WAY IS THE NAME GIVEN TO THE FAINT BAND OF LIGHT that stretches
across the night sky. This light comes from stars and nebulae in our galaxy, known as
the Milky Way Galaxy or simply as “the Galaxy.” The Galaxy is believed to be a barred
spiral, with a dense central bar of stars encircled by four arms spiraling outward and
surrounded by a less dense halo. We cannot see the spiral shape because the solar system
is in one of the spiral arms, the Orion Arm (also called the Local Arm). From our
position, the center of the Galaxy is completely obscured by dust clouds; as a result, optical
maps give only a limited view of the Galaxy. However, a more complete picture can be
obtained by studying radio, infrared, and other radiation. The central part of the Galaxy is relatively small
and dense and contains mainly older red and yellow stars. The halo is a less dense region in which the oldest
stars are situated; some of these stars are as old as the Galaxy itself (possibly 13 billion years). The spiral
arms contain main sequence stars and hot, young, blue stars, as well as nebulae (clouds of dust and
gas inside which stars are born). The Galaxy is vast, about 100,000 light-years across (a light-year is about
5,870 billion miles/9,460 billion km); incomparison, the solar system seems small, at about 12 light-hours
across (about 8 billion miles/13 billion km). Theentire Galaxy is rotating in space, although
the inner stars travel faster than those farther out. The Sun, which is
about two-thirds out from the center, completes one lap of the Galaxy
about every 220 million years.
SIDE VIEW OF OUR GALAXY
OVERHEAD VIEW OF OUR GALAXY
VIEW TOWARD
GALACTIC CENTER
PANORAMIC OPTICAL MAP OF OUR
GALAXY AND NEARBY GALAXIES
THE MILKY WAY
15
Dust clouds
obscuring
Galactic center
South Galactic Pole
Small Magellanic Cloud, an irregular
galaxy 190,000 light-years away
Large Magellanic Cloud, an irregular galaxy 170,000 light-years
away; one of the nearest galaxies to the Milky Way
Canopus, a white supergiant;
thesecond brightest star in the sky
Sirius, a white main
sequence star; the
brightest star in the sky
Orion Nebula
Orion’s belt,
a row of
three bright
stars
Galactic
plane
Light from stars
and nebulae in the
Perseus Arm
Light from stars and nebulae in the
part of the Sagittarius Arm between
the Sun and Galactic center
Dark clouds of dust and gas
obscuring light from part of the
Sagittarius Arm
North Galactic Pole
Vega, a white main
sequence star; the fifth
brightest star in the sky
Blue indicates
low-intensity
radio-wave emission
South
Galactic Pole
Yellow and green
indicatemedium-intensity
radio-wave emission
South Galactic Pole
High-intensity infrared
radiation from interstellar
gas and dust
Galactic
plane
Galactic
plane
Galactic
plane
North Galactic spur
(possibly radio emission
from a supernova remnant)
North Galactic
Pole
Red indicates
high-intensity
radio-wave emission
North Galactic
Pole
Low-intensity infrared
radiation from interstellar
gas and dust
High-intensity
infrared
radiation
fromregion
ofstarbirth
PANORAMIC INFRARED MAP OF OUR GALAXY
PANORAMIC RADIO MAP OF OUR GALAXY
Galactic
plane
A NEBULA IS A CLOUD OF DUST AND GAS inside a galaxy. Nebulae become visible if the
gasglows, or if the cloud reflects starlight or obscures light from more distant objects.
Emission nebulae shine because their gas emits light when it is stimulated by radiation
fromhot young stars. Reflection nebulae shine because their dust reflects light from stars
in or around the nebula. Dark nebulae appear as silhouettes because they block out light
from shining nebulae or stars behind them. Two types of nebula are associated with dying
stars: planetary nebulae and supernova remnants. Both consist of expanding shells of gas
that were once the outer layers of a star. A planetary nebula is a gas shell drifting away
from a dying stellar core. A supernova
remnant is a gas shell moving away from a stellar
core at great speed following a violent explosion
called a supernova (see pp. 26-27). Stars are often
found in groups known as clusters. Open clusters
are loose groups of a few thousand young stars
that were born from the same cloud and are
drifting apart. Globular clusters are densely
packed, roughly spherical groups of hundreds
of thousands of older stars.
THE UNIVERSE
16
Nebulae and star clusters
HODGE 11, A
GLOBULAR CLUSTER
Wisps of dust and
hydrogen gas. The
cluster is passing
through a region of
interstellar
material
Glowing filament
of hot, ionized
hydrogen gas
Alnitak (star in
Orion’s belt)
Dust lane
Emission nebula
Starbirth region
(area in which
dust and gas
clump together
to form stars)
Dust lane
PLEIADES (OPEN STAR CLUSTER)
WITH A REFLECTION NEBULA
Star near southern
end of Orion’s belt
Emission nebula
Horsehead Nebula
Reflection nebula
Dark nebula
obscuring light
from distant stars
Emission
nebula
Reflection
nebula
TRIFID NEBULA (EMISSION NEBULA)
HORSEHEAD NEBULA (DARK NEBULA)
Young star in an
open cluster of more
than 1,000 stars
Reflection nebula
NEBULAE AND STAR CLUSTERS
17
Glowing
cloud of dust
and hydrogen
gas forming
part of Orion
Nebula
Dust cloud
Trapezium
(group of four
young stars)
Red light
from hot,
ionized
hydrogen gas
HELIX NEBULA (PLANETARY NEBULA)
Planetary nebula
(gas shell expanding
outward from dying
stellar core)
Core remnant with
surface temperature
of about 180,000°F
(100,000°C)
Red light from
hot, ionized
hydrogen gas
Blue-green light from
hot, ionized oxygen
and nitrogen gases
Glowing
filament of
hot, ionized
hydrogen gas
Hydrogen gas
emitting red
light due to
being heated
by supernova
explosion
Supernova
remnant (gas
shell consisting
of outer layers
of star thrown
off in
supernova
explosion)
VELA SUPERNOVA REMNANT
Green light
from hot,
ionized
oxygen gas
Glowing
filament of
hot, ionized
hydrogen gas
Gas cloud
emitting
lightdue to
ultraviolet
radiation from
the four young
Trapezium stars
ORION NEBULA (DIFFUSE EMISSION NEBULA)
THE UNIVERSE
18
Chi
2
Orionis
Xi Orionis
Mu Orionis
Betelgeuse
Orion’s belt
Alnitak
Saiph
Orion
Nebula
Alnilam
Rigel
Tau Orionis
Eta Orionis
Mintaka
Pi
6
Orionis
Pi
5
Orionis
Pi
4
Orionis
Pi
3
Orionis
Pi
2
Orionis
Omicron
Orionis
Chi
1
Orionis
Nu Orionis
Heka
Bellatrix
VISIBLE STARS IN THE NORTHERN SKY
Stars of northern skies
WHEN YOU LOOK AT THE NORTHERN SKY, you look away from the densely
populated Galactic center, so the northern sky generally appears less
bright than the southern sky (see pp. 20-21). Among the best-known
sights in the northern sky are the constellations Ursa Major
(the Great Bear) and Orion. Some ancient civilizations believed
that the stars were fixed to a celestial sphere surrounding the
Earth, and modern maps of the sky are based on a similar
idea. The North and South Poles of this imaginary
celestial sphere are directly above the North and South
Poles of the Earth, at the points where the Earth’s axis
of rotation intersects the sphere. The celestial North
Pole is at the center of the map shown here, and
Polaris (the North Star) lies very close to it. The
celestial equator marks a projection of the
Earth’s equator on the sphere. The ecliptic
marks the path of the Sun across the sky
as the Earth orbits the Sun. The Moon and
planets move against the background of
the stars because the stars are much more
distant; the nearest star outside the solar
system (Proxima Centauri) is more
than 50,000 times farther away than
the planet Jupiter.
ORION
L
U
P
U
S
L
E
P
U
S
Antares
Zubeneschamali
Spica
Arcturus
Denebola
Alioth
Kochab
Dubhe
Algieba
Regulus
Praesepe
Precyon
Castor
Capella
Pollux
Alhena
El Nath
Aldebaran
Rigel
Betelgeuse
Strius
Mirzam
Wezen
Adhara
Gamma
Velorum
Alphard
Cor
Caroli
Alkaid
Alphecca
Zubenelgenubi
L
I
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R
A
V
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G
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STARS OF NORTHERN SKIES
19
Almach
Mirach
Delta
Andromedae
Alpheratz
Algenib
Markab
Omicron
Andromedae
Lambda
Andromedae
Theta
Andromedae
Andromeda
Galaxy
Nu Andromedae
Phi Andromedae
51 Andromedae
Mu Andromedae
Scheat
Xi Pegasi
Matar
Mu Pegasi
Iota Pegasi
Pi Pegasi
Hamal
Kappa Pegasi
Lambda Pegasi
Enif
Theta
Pegasi
Dubhe
Merak
Phekda
Megrez
Alioth
Alkaid
Mizar
Alcor
PEGASUS AND ANDROMEDA
THE BIG DIPPER, PART OF URSA MAJOR (THE GREAT BEAR)
Shaula
Kaus
Australis
Nunki
Altair
Ras
Alhague
Vega
Eltanin
Alderamin
Polaris
Deneb Enif
Scheat
Markab
Fomalhaut
Deneb
Algedi
Al
Nair
Algenib
Nair Al
Zaurak
Deneb
Kaitos
Mira
Acamar
Alpheratz
Schedar
Mirfak
Mirach
Hamal
Pleiades
Menkar
Almach
Algol
S
C
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P
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C
O
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THE UNIVERSE
20
Small
MagellanicCloud
Beta Hydri
Gamma
Hydri
Gamma Mensae
Alpha Mensae
Eta Mensae
Beta Mensae
Large Magellanic
Cloud
Epsilon Hydri
Delta Hydri
Alpha
Hydri
Stars of southern skies
WHEN YOU LOOK AT THE SOUTHERN SKY, you look toward the Galactic
center, which has a huge population of stars. As a result, the Milky
Way appears brighter in the southern sky than in the northern sky
(see pp. 18-19). The southern sky is rich in nebulae and star
clusters. It contains the Large and Small Magellanic Clouds,
which are two of the nearest galaxies to our own. Stars make
fixed patterns in the sky called constellations. However,
the constellations are only apparent groupings of stars,
since the distances to the stars in a constellation may
vary enormously. The shapes of constellations may
change over many thousands of years due to the
relative motions of stars. The movement of the
constellations across the sky is due to the Earth’s
motion in space. The daily rotation of the Earth
causes the constellations to move across the
sky from east to west, and the orbit of the
Earth around the Sun causes different areas
of sky to be visible in different seasons. The
visibility of areas of sky also depends on
the location of the observer. For instance,
stars near the celestial equator may be
seen from either hemisphere at some
time during the year, whereas stars
close to the celestial poles (the celestial
South Pole is at the center of the map
shown here) can never be seen from
the opposite hemisphere.
HYDRUS (THE WATER SNAKE) AND
MENSA (THE TABLE)
VISIBLE STARS IN THE SOUTHERN SKY
H
E
R
C
U
L
E
S
L
Y
R
A
Vega
Albireo
Altair
Nunki
Ras Alhague
Ras Algethi
A
Q
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P
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C
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